This invention relates generally to pollution control systems and, more particularly, to a pollution control system for the removal of sulfur oxides from flue gases and the reduction of the sulfur oxides to sulfur.
It is known in the field of atmospheric pollution control to use an adsorptive process for the desulfurization of flue gases in which the sulur-containing material is adsorbed in the porous system of an activated carbonaceous material. In one such process, adsorption is carried out in a gas-solid contacting device in which the flue gases are contacted with activated char and sulfur dioxide in a diluted form in the gas stream passing through the activated char is adsorbed and oxidized to sulfuric acid by the oxygen and water vapor present in the gas stream. Other acid gases, such as nitrogen oxides, are similarly adsorbed, and particulate matter entrained in the gas stream is filtered by passage of the stream through the activated char.
In the prior art process developed by Bergbau Forshung and depicted in FIG. 1, the acid-laden or saturated char is thermally regenerated in a desorption vessel, or the like, by a process in which the sulfur-containing material is chemically changed in form, resulting in the decomposition of sulfuric acid to sulfur dioxide and water, whereby a portion of the carbonaceous adsorbent is oxidized to carbon dioxide. The by-product of the regeneration process is a gas stream containing 20-30% by volume of sulfur dioxide, which is directed to an off-gas treatment facility for further processing.
The prior art system of FIG. 1 is composed of three basic subsystems: adsorption, regeneration or desorption, and off-gas treatment. In the adsorption subsystem, an adsorber 10 receives flue gases from the vapor generator after they have passed through a particulate matter separator, or the like (not shown), and the flue gases are contacted with adsorbent material in pellet form loaded into the adsorber. The adsorbent material used in the adsorber 10 is usually in the form of a preoxidized bituminous coal, or activated char.
In the regeneration subsystem, an inert, heat exchange medium, such as sand, is heated in a sand heater 12 to a predetermined, elevated temperature, and is supplied to a regenerator 14, through which the heated sand and the saturated char pellets pass in intimate contact. This contact raises the temperature of the mixture to a predetermined level to cause the sulfuric acid in the porous system of the activated char to be converted first to sulfuric acid anhydrate (H.sub.2 SO.sub.3) and then to SO.sub.2, and the nitrogen compounds to N.sub.2. A high-concentrated, SO.sub.2 -rich off-gas stream is produced, containing 20-30% by volume, and is usually directed to an exterior unit for further processing. The sand/char mixture leaving the regenerator 14 goes through a separator 16, which separates the regenerated char from the sand. The separated sand is returned to the sand heater 12, again heated to the proper elevated temperature, and recycled into the regenerator 14. The separated char is directed to a char cooler 18, in which it is cooled and recycled to the adsorber 10 for re-use.
In this type of prior art system, the SO.sub.2 -rich off-gas is usually treated further to produce elemental sulfur, which is storable and which has certain commercial applications. To this end the off-gas can be reacted with crushed coal to produce the elemental sulfur. For example, in the system disclosed in the above-identified application, the SO.sub.2 -rich off-gas is introduced into a reactor, shown in general by the reference numeral 20 in FIG. 1, and is initially reacted with crushed coal which is continuously supplied to the vessel to yield gaseous elemental sulfur, which is then passed to a condenser 22 and condensed into liquid sulfur. The liquid sulfur may be stored in appropriate containers, or may be cooled into solid form.
In the above-described system of FIG. 1, a certain quantity of the activated char is consumed by the chemical reaction which occurs in the regenerator 14, and as a result of the continuous recyling of the regenerated char, a portion of this material becomes physically reduced to such a size which renders it ineffective in the adsorption process. Thus, a source of additional activated char is provided to the adsorber 10 to replenish the char consumed in the regenerator 14 and to make up for the quantity which is physically reduced. (In the reactor 20, the crushed coal supplied thereto is consumed in the reaction process, yielding coal ash as a by-product, which is not otherwise utilized in the process.)
The major problem with the prior art process of FIG. 1 resides in the inherent difficulty in separating the regenerated adsorbent from the sand for recycle to the adsorber. Also, with increased emphasis on energy and natural resource conservation, a need exists for replacing that commercial process with a process which is more energy efficient, specifically, one which achieves regeneration with like efficiency at lower temperature.
It is also well known in the art to regenerate activated carbons by contact with a hot regenerating gas. For example, U.S. Pat. No. 2,933,454 discusses regeneration with mixtures of air and steam. However, where it is desirable to treat the desorbed gas for recovery, for example, of sulfur value, regeneration of the adsorbent by contact with a hot regenerating gas poses a problem of dilution of the desorbed volatiles to the point where they cannot be economically treated for the recovery of some value contained therein. One solution to this problem is disclosed in U.S. Pat. No. 3,667,910 wherein it is taught that the regenerating gas may be recirculated through the desorber to elevate the concentration of sulfur dioxide to facilitate recovery (Col. 4, lines 40-45). The problem with this prior art process is that to achieve adequately effective regeneration, carbon monoxide gas and/or hydrogen gas must be continuously generated and combined with the circulating desorbent.